The present invention relates to a packaging system for a thyristor, and more particularly to such a system providing electrical and thermal connections to a thyristor.
Thyristors are semiconductor devices used for switching power level currents. A thyristor typically has a flat, pellet shape and comprises silicon bonded to a backing plate typically formed of tungsten or molybdenum. Such a thyristor has a cathode on the exterior, flat surface of the silicon and the backing plate serves as an anode. In operation, thyristors become heated to an extent to which is dependent upon the amount of current they carry. The maximum current which a particular thyristor can carry is limited by the maximum temperature which the thyristor can withstand. In order to facilitate removal of heat from a thyristor as well as to provide good electrical contact thereto, the anode and cathode of the thyristor typically comprise substantial surface area. The higher the current carrying rating of the thyristor is, the larger these surface areas are required to be. For a pellet-shaped thyristor above about 12 mm in diameter, the attachment of power conductors to the anode and the cathode thereof is problematic. Suitable power conductors having desirable, high thermal conductivity (for heat removal) such as solid copper have a much higher thermal expansion rate than does silicon. The solder bonding of solid copper to the silicon of such a thyristor, which is feasible for smaller diameter thyristors, is accordingly subject to significant stresses between the solid copper and the silicon. These stresses can result in either the failure of a solder bond or shearing of the silicon.
A known means for attaching power conductors to the anode backing plate and cathode of a thyristor comprises a solid molybdenum strain buffer abutting the cathode of the thyristor, but not bonded thereto, and nickel-coated copper abutting the anode backing plate of the thyristor, but likewise not bonded thereto. Because the interfaces between the foregoing parts are not bonded, they are referred to as "dry interfaces." (This is so whether or not the interfaces are provided with a thermal grease.) The known means further includes bulky structure (that is, large and heavy structure) for pressing the foregoing parts together with a large force. Such force is necessary to provide acceptable electrical and thermal conductivities across the dry interfaces while accommodating the different rates of thermal expansion between the silicon and the power conductor attached to the cathode (which is located on the silicon). Although such electrical and thermal conductivities are usually "acceptable", they typically underutilize the current carrying capacity of the thyristor. It would be desirable to provide a packaging system for a thyristor which does not require a bulky structure for holding the power conductors to the anode and cathode thereof. It would also be desirable to provide a packaging system for a thyristor which utilizes bonds for connection of power conductors to both the anode and cathode of the thyristor rather than dry interfaces.